The present invention relates generally to an improved system for cleaning wafers after going through chemical mechanical polish (CMP).
During manufacturing stages of an integrated circuit (IC), chemical mechanical polish (CMP) is commonly used for removing any unnecessary materials. CMP is considered to be a unique fabrication process, because a wafer is deliberately introduced into an environment with a high particle count. During the CMP process, a polishing slurry that is full of abrasive particles in the nanometer to micrometer scale is used. Once the wafer has finished polishing, work must be done to carefully remove those particles from the wafer surface or they will affect the further fabrication steps.
In order to remove those particles from the wafer surface, such that the possibility of defects can be minimized, a cleaning process, typically referred to as post-CMP cleaning, is necessary. After CMP, the wafer must be kept wet prior to cleaning because once the slurry is allowed to dry on the wafer, it becomes hard to remove. Due to electrostatic attraction forces, a simple water rinse may remove little, if any, of these particles after polishing. Consequently, conventional systems using a set of wafer brush cleaners were developed to clean and dry the wafer after CMP. These conventional systems typically use a set of PVA brushes to mechanically wipe the surfaces of the wafer and remove the abrasive particles. In the conventional systems, the wafer is typically cleaned by placing the wafer between a pair of spinning PVA brushes while the wafer itself is being rotated by a set of roller caps which are placed on the bottom of a brush module. The wafer in the brush module has forces exerted on it in two directions: one is exerted by the pair of rotating PVA brushes; the other is applied by the rotating roller caps. A friction force is exerted on the surface of the wafer by the pair of PVA brushes as they scrub in the down direction. This force will push and press the wafer down to contact the roller caps. Other tangent forces are applied by the rotating roller caps at the bottom of the brush module to make the wafer rotate by itself. These tangent forces must be greater than the friction force on the wafer surface exerted by the PVA brushes. Otherwise, the wafer will not rotate smoothly, and may stop at any time, thereby causing the slurry to reside in a certain area. This defect is known as a fan type defect. Since no tools are implemented in such a conventional system to measure the rotation speed and direction of the wafer and PVA brushes, fan type defects and an unstable etch rate can occur during the post-CMP cleaning process, thereby lowering the yield and efficiency in reducing defects.
It is therefore desirable to have a new improved system for post-CMP cleaning such that the yield and efficiency can be improved and that the abnormal wafer recovery success rate may be increased.